It is known that differences in thermal properties of the die and the metallic die attach pad may produce large stresses, particularly in a silicon die after attachment to the die attach pad. These stresses may significantly affect the electrical properties of the silicon die, and die stress contributes to substantial yield and reliability problems, particularly in linear semiconductor devices.
A conventional die attach process requires bonding of a silicon die, for example, to a metallic die attach pad with a polyimide bonding agent. The bonded die assembly must be subjected to elevated temperatures for several hours to properly cure the bonding agent and provide good adhesion between the silicon die and the die attach pad. Upon cooling of the bonded die assembly to ambient operating temperatures, differences in the thermal properties of the silicon die and the metallic die attach pad produce stresses in the die. Linear semiconductor devices are especially sensitive to small die stress shifts on the order of about 5% and less, since small die stress shifts produce substantial alterations in electrical properties, particularly resistance. The die stress from a standard polyimide die attach process may introduce shifts of up to 10% in circuit parameters which poses serious yield and reliability problems. Since the stress shift is usually non-uniform over the die, devices on different parts of the die will also not track one another in their changes. Significant lack of precision in the relative operating parameters of the devices thus results, in addition to the overall shift in parameters induced by stresses.
Problems relating to bonding materials having different thermal properties have been addressed in a variety of applications. U.S. Pat. No. 3,284,176 teaches brazing waffle-like grooved surfaces of a metallic member to a metallized ceramic surface to overcome thermal stresses caused by differential coefficients of thermal expansion of the ceramic and the metallic member. Thermal stresses are presumed to be relieved in the transitional area between the tops of the grooved surfaces and the main body of the metallic member. U.S. Pat. No. 4,211,354 teaches a method for alleviating stress damage in metallic matrix composites by positively introducing discontinuities at the interface between the layers to reduce thermal stress produced by unequal thermal expansion of the layers. The '354 patent teaches drilling holes or forming grooves in a grid pattern in the metallic matrix layer to provide a discontinuous bond at the interface between the layers.
One approach specific to reducing die stress resulting from differences in thermal properties of the silicon die and the metallic die attach pad is to reduce adhesion between the die and the die attach pad. This approach has been unsatisfactory, however, since reduced adhesion results in reduced reliability of the die assembly. Modifications to the structure of the die attach pad have also been proposed for a variety of purposes. For example, U.S. Pat. No. 4,445,271 teaches a perforated ground pad in the form of a grid bonded to a ceramic substrate and connected to a support by means of tabs. The ground pad is provided with perforations for passage of gases which may form during bonding of the surface of the ground pad to the ceramic surface. U.S. Pat. No. 4,451,973 teaches a plastic encapsulated semiconductor device and leadframe therefor wherein the substrate support of the leadframe may be provided with a three-dimensional patterned surface to increase the surface area of contact between the substrate support and plastic encapsulation' material. The three-dimensional patterned surface of the leadframe also increases adherence and heat dissipation.
None of the prior methods and apparatus has satisfactorily overcome the problems related to die stresses caused by differential thermal coefficients of expansion of the silicon die and the metallic die attach pad while maintaining good adhesion and shear strength of the die assembly.
Accordingly, it is an objective of the present invention to provide a method and apparatus for reducing die stress in a die assembly resulting from differential thermal properties of the silicon die and the metallic die attach pad.
It is another objective of the present invention to maintain good adhesion and shear strength in a die assembly including a die bonded to a metallic die attach pad, while reducing die stress caused by differential thermal properties of the die and the die attach pad.
It is yet another objective of the present invention to provide a die attach pad which permits the die to contract during the die attach process by deforming the die attach pad to relieve die stress in the bonded die assembly.